Determine Cloud SDK (gcloud) version update strategy

[busunkim96]

Some images always install the latest version of Cloud SDK. Others use a pinned version.

Make a decision as to which option is more appropriate and make this consistent across the images.

Latest (using installer):

testing-infra-docker/java/java9/Dockerfile

Line 37 in 5da8b1c

curl -sSL https://sdk.cloud.google.com | bash -s -- --install-dir=/usr/local/gcloud

Pinned (using versioned archive):

testing-infra-docker/python/cloud-devrel-kokoro-resources/python/Dockerfile

Lines 42 to 48 in b604af8

	ENV CLOUD_SDK_VERSION 253.0.0
	# Use system python for cloud sdk.
	ENV CLOUDSDK_PYTHON /usr/bin/python
	RUN wget https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud-sdk-$CLOUD_SDK_VERSION-linux-x86_64.tar.gz
	RUN tar xzf google-cloud-sdk-$CLOUD_SDK_VERSION-linux-x86_64.tar.gz
	RUN /google-cloud-sdk/install.sh
	ENV PATH /google-cloud-sdk/bin:$PATH

Installation guide:
https://cloud.google.com/sdk/docs/downloads-interactive